Hydrocarbon conversion processes, particularly alkylation of aromatic hydrocarbons, are the foundation for the production of an assorted variety of useful petrochemicals. For instance, ethylbenzene and ethyltoluene, as well as other alkyl-substituted aromatics, are beneficial as feedstocks for the production of a variety of styrenic polymer materials, cumene, and detergent alkylates. Alkyl-substituted aromatics are also useful as high octane transportation fuels. The use of alkyl-substituted aromatics as blending agents for gasoline expands product volume and increases octane values. Aromatic alkylation processes also provide an economic method of reducing benzene content in gasoline.
In the past, alkylation processes have employed acidic catalysts such as AlCl.sub.3, FeCl.sub.3, SbCl.sub.5, BF.sub.3, ZnCl.sub.2, TiCl.sub.4, HF, H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, P.sub.2 O.sub.5 and the like. Reactions using these catalysts are generally carried out at low temperatures and, in particular, when a Friedel-Craft catalyst is employed, in the presence of a hydrogen halide, such as HCl.
There are numerous problems associated with the use of these acidic catalysts in an alkylation process. First, these catalysts are very corrosive, thereby requiring the use of exotic materials for process equipment. Second, the catalyst consumption and the regeneration costs are high. Third, the yields of alkylate boiling in the gasoline range are low. Fourth, complicated separations and recycle of feed can be required. Fifth, these catalysts tend to polymerize the olefinic reagents, thus minimizing available starting materials.
Many of these problems have been avoided by using synthetic or natural zeolite materials as alkylation catalysts. Zeolitic materials are porous crystalline aluminosilicates having a definite crystalline structure within which there are a large number of smaller cavities which can be interconnected by a number of even smaller channels. Since the dimensions of these pores are such that molecules of a certain dimension are accepted for adsorption while larger molecules are rejected, these materials have come to be known as "molecular sieves."
A particular type of molecular sieve useful in aromatic alkylation reactions is a crystalline aluminosilicate zeolite. Crystalline aluminosilicate zeolites are composed of a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, such as an alkali metal or an alkaline earth metal. Since these cations are exchangeable, it is possible to vary the properties of a particular aluminosilicate by selection of a suitable cation.
The use of crystalline aluminosilicates in aromatic alkylation processes is well known in the art. In U.S. Pat. No. 2,904,607 there is disclosed a process for alkylating aromatics with olefins in the presence of a crystalline metallic aluminosilicate having a uniform pore opening of about 6-15 Angstroms. In U.S. Pat. No. 3,251,897 there is disclosed a process for alkylating aromatics with a crystalline aluminosilicate which contains rare earth metal cations and a uniform pore volume of at least 6 Angstroms.
While the use of crystalline aluminosilicate zeolite catalysts in aromatic alkylation processes represents a distinct improvement over Friedel-Craft catalysts, zeolite catalysts have the disadvantage of producing unwanted quantities of impurities. They also deactivate at a rapid rate, particularly in vapor phase reaction zones. Consequently, the industry began treating zeolites to specifically address these deactivation and selectivity problems. In U.S. Pat. No. 2,897,246, there is disclosed heating a crystalline aluminosilicate in the presence of water vapor at a temperature of 400-900 deg C prior to using it in an alkylation process. In U.S. Pat. No. 3,631,120, there is disclosed ammonium-exchanging a crystalline aluminosilicate to achieve a silica-to-alumina molar ratio of 4.0-4.9 prior to using it in an aromatic alkylation process. Other methods of treating a crystalline aluminosilicate for use in an aromatic alkylation process include steaming and ammonium-exchanging the zeolite (3,641,177), rare earth-exchanging the zeolite in the presence of sulfur dioxide (4,395,372), partially collapsing the zeolite to reduce crystallinity (4,570,027), contacting the zeolite with aluminum chloride vapor (4,575,573), increasing the total amount of lattice metal in the zeolite (4,665,255), reacting the zeolite with an acidic inorganic oxide in the presence of water (4,665,253 ), or depositing carbonaceous material on the zeolite (4,798,816).
New crystalline aluminosilicates were also developed to address these selectivity and deactivation problems. U.S. Pat. Nos. 4,393,263, 4,291,185, 4,387,259, 4,393,262, and 4,469,908 disclose the use of ZSM-12 in an aromatic alkylation process. U.S. Pat. No. 4,547,605 discloses the use of ZSM-23 in an aromatic alkylation process. U.S. Pat. No.4,717,780 discloses the use of ZSM-58 in an aromatic alkylation process.
U.S. Pat. No. 4,185,040 discloses that the selectivity and deactivation problems associated with the use of crystalline aluminosilicates can be addressed by shaping the extrudates to give a high ratio of external surface area to crystal pore volume. Crystal pore diameter is defined as 5-15 Angstroms.
The process conditions used during aromatic alkylation can affect the performance of a crystalline aluminosilicate zeolite catalyst. For example, vapor phase aromatic alkylation processes, such as those disclosed in U.S. Pat. Nos. 3,751,504 and 3,751,506, generally have high conversions due to greater ease in the diffusion of the vapor reactants into the micropores of the crystalline aluminosilicate, but high catalyst deactivation rates due to olefins attaching to the active sites of the zeolite catalyst and deactivating the catalyst by coke deposition. On the other hand, liquid phase aromatic alkylation processes, such as those described in U.S. Pat. Nos. 3,251,897 and 3,631,120, generally have lower conversion rates due to diffusional limitations, and also lower catalytic deactivation rates due to aromatics preferentially occupying the active sites of the zeolite catalyst, thereby preventing the olefins from deactivating the catalyst.
It is therefore desirable to provide an improved aromatic alkylation process which overcomes most, if not all, of the preceding problems.